There is known in the art a class of devices generally referred to as “scroll” vacuum pumps, compressors and expanders, together referred to as positive fluid displacement apparatus, wherein two interfitting spiroidal or involute spiral elements are conjugate to each other and are mounted on separate end plates forming what may be termed as fixed and orbiting scrolls. These elements are interfitted to form line contacts between spiral elements.
A pair of adjacent line contacts and the surfaces of end plates form at least one sealed off pocket. When one scroll, i.e. the orbiting scroll, makes relative orbiting motion, i.e. circular translation, with respect to the other, the line contacts on the spiral walls move along the walls and thus changes the volume of the sealed off pocket. The volume change of the pocket will expand or compress the fluid in the pocket, depending on the direction of the orbiting motion. When the volume change of the pocket compresses the fluid in the pocket, a pressure is created inside the pocket such that a separating force in the axial direction is generated between the fixed and orbiting scrolls. This phenomenon can cause low machine efficiency.
Referring to U.S. Pat. No. 6,224,059, there is a scroll type compressor in which two separate seal structures are provided. The seal structures are positioned radially inward and outward behind the orbiting scroll member so as to create a chamber. The chamber is allowed to receive pressurized gas, which generates a back pressure force.
The seal structure includes a seal jacket having a rear wall and inwardly extending lips, and a coil spring positioned inside the seal jacket. The coil spring, together with the pressurized gas that is leaked into the chamber, provides a back pressure force that forces the lip to press against the rear surface of the orbiting scroll. However, the high contact force against the lip seal can lead to undue wearing of the seal.
U.S. Pat. No. 6,224,059 discloses a seal structure that is further provided with a sheet on the lip facing the orbiting scroll. The surface of the sheet is provided with a tab extending outwardly toward the orbiting scroll. The tab provides a contact area for the rear surface of the orbiting scroll. The tab reduces the total contact force experienced between the seal and the rear surface of the orbiting scroll by reducing the force imbalance due to the pressure gradient along the lip. However, in practice, it does not take long for frictional wear of the sealing tabs, especially in scroll compressors with an oil-free design, to render the sealing tabs ineffective.